The transmission capacity of optical communications systems is presently limited by the optical source modulation bandwidth and dispersive and nonlinear propagation effects. Although a span of optical fiber has a very broad optical bandwidth (10-20 THz), the system data rates transmitted over such spans are presently limited to about 2.5 Gbits/sec in single-channel communications systems. Wavelength division multiplexing (WDM) generally increases optical system capacity by simultaneously transmitting data on several optical carrier signals at different wavelengths. The total system capacity is increased by a factor equal to the number of different wavelength channels. Other advantages of WDM are realized in point-to-multipoint communications systems such as in fiber-to-the-home. In this case, improved power splitting budget, security, upgradability, service flexibility and lower component speed requirements compared to time-division-multiplex (TDM) point-to-point links make WDM attractive.
WDM systems which have heretofore been proposed generally include a separate optical modulation source for each optical channel or individual transmission wavelength. For example, an array of laser diodes may be used--with each laser diode being tuned to a different frequency and modulated individually. The laser frequencies are combined as, for example, by an optical coupler and are then launched into one end of an optical fiber. At the other end of the fiber, the wavelength channels are separated from one another and directed to corresponding receivers.
Due to a number of technical problems, presently proposed WDM systems are not regarded as being commercially viable for mass market applications like fiber distribution to the home. One such problem is the small number of channels currently accommodated. Specifically, while a WDM system would be considered cost-effective if a large number of channels (32-64 or even 128) were made available, present multi-channel laser diodes are very difficult to fabricate with acceptable yield even with as few as 8 channels. In addition, passive WDM splitters currently available have a large temperature variation of their passband channels, thereby requiring a continuous tunability in the multichannel sources that has not yet been achieved.
Therefore, although WDM offers an elegant solution to increasing the capacity and transparency of optical networks, WDM for fiber distribution networks as currently envisioned is not deemed to be cost-competitive with simple point-to-point schemes (one fiber per customer), and more cost-effective schemes are needed. For fiber-to-the home optical communications systems, low-cost methods of delivering optical signals into and out from the home is a challenging problem. Although time-domain multiplexing (TDM) of data streams would be another method of increasing transmission capacity, it is not desirable to build a specific network with expensive high frequency electronic components that are difficult to upgrade in the future. For example, in order to deliver 50 Mbits/sec data rates into a single house, a 32 channel system would require transmitters, routers, amplifiers, receivers and modulators with 1.5 Gbits/sec capacity and above. It is not desirable to place such expensive and state-of-the-art components into every home. In addition, it is desirable to have as much of the system in the field and in the home transparent and passive, i.e. line-rate independent and not requiring any electrical powering. In addition to the low data rate systems as required for local access (50-155 MHz), high data rate systems (622 MHz-2.5 Gbits/sec) can also benefit from WDM. In such a case, similar problems are caused by the difficulty in obtaining a multifrequency source with adequate channel tuning, stability and modulation bandwidth.
As is apparent from the above, there is a continuing need for an efficient and cost-effective WDM system that is capable of transmitting a large number of spectral channels.